The present invention relates to a process for reducing halations in an irradiated reproduction layer by adding an auxiliary agent to the reproduction layer.
A support material for offset printing plates is provided, either directly by the user or by the manufacturer of precoated printing plates, with a radiation-sensitive layer (reproduction layer) on one or both sides of the support material, with the aid of which layer a printable image of an original is produced by photomechanical means. After the production of a printing form from the printing plate, the support carries the image areas which are ink receptive in the subsequent printing and, at the same time, forms the hydrophilic image background for the lithographic printing process in the areas which are image-free (non-image areas) during subsequent printing.
It is technically feasible to use foils of aluminum, steel, copper, brass or zinc and also plastic films or paper as the base materials for supports of this kind. These raw materials are converted into supports of offset printing plates by appropriate modifying treatments, for example, graining, dull chromium plating, superficial oxidation and/or application of an intermediate layer. Aluminum, which is presently the most widely used base material for offset printing plates, is superficially roughened according to known methods, e.g., by dry brushing, slurry brushing, sandblasting, chemically and/or electrochemically treating. To increase resistance to abrasion, the roughened substrate can additionally be subjected to an anodization, in which a thin oxide layer is formed on the substrate.
The printing plate support coated with a radiation-sensitive layer must meet additional requirements, some of which are correlated with the requirements demanded of the support material itself. These requirements include, for example, high radiation sensitivity (photosensitivity), good developability, clear contrast after exposure and/or development, large print runs and a reproduction which is, as closely as possible, true to the original. In this connection, the ability of a radiation-sensitive layer to remain substantially free of halation (i.e., the lateral and oblique spreading of radiation through the layer) upon irradiation (exposure) of the printing plate also gains increasing importance, particularly in printing plates provided with positive-working radiation-sensitive layers. From the prior art, the following publications are, for example, known that describe attempts to solve the halation problem.
In German Offenlegungsschrift No. 25 12 043 (corresponding to U.S. Pat. No. 4,168,979), a radiation-sensitive printing plate is described in which the surface of the radiation-sensitive layer is coated with a matt layer that is removed by developing the exposed plate. This matt layer usually is a binder layer (e.g., comprising a cellulose ether), having matting particles, such as particles of SiO.sub.2, ZnO, TiO.sub.2, ZrO.sub.2, glass, Al.sub.2 O.sub.3, starch or polymers dispersed therein. It is stated that a printing plate built up in this manner reduces the period of time required to attain a largely complete and uniform contact between the film original and the radiation-sensitive layer during the exposure step of the platemaking process.
German Offenlegungsschrift No. 29 26 236 (corresponding to Canadian Pat. No. 1,148,014) discloses a radiation-sensitive reproduction material. The positive-working radiation-sensitive layer of this material contains particles, corresponding to the matting particles disclosed in the above-mentioned German Offenlegungsschrift No. 25 12 043, the smallest dimension of which particles is at least equal to the thickness of the layer itself. This type of material is intended for any application in which positive contact copies must be made in a vacuum copying frame and in which it is important to obtain a high image resolution and a true reproduction of the original. In particular, it is stated that the material shows a reduced halation tendency in the copying procedure. Halation may occur during irradiation due to a locally increased distance between the original and the light-sensitive layer, resulting in an imprecise reproduction of small image elements, such as screen dots.
Applying particles to the radiation-sensitive layer, together with a binder, or incorporating particles in the layer without any special binder is, however, an expensive process step which requires a great deal of care, particularly in modern, continuously-operating coating lines. Moreover, in developing the layer the particles which have been applied or added to the layer form a sort of "foreign matter" in the developing liquid and, particularly, in the automatically working developing apparatus as well, leading to interference with the functioning of the apparatus.
Published European Patent Application No. 42,104 (corresponding to U.S. Pat. No. 4,510,227) discloses adding surface-active polysiloxanes to reproduction layers in order to achieve a more uniform coating. The polysiloxanes used contain dialkyl siloxane units as well as alkylene oxide units. Although an effect of such additions on the halation behavior of the reproduction layers is mentioned, it is not specified in detail.